The present invention generally relates to copying apparatus and more particularly, to an electrophotographic copying apparatus provided with a control mechanism for maintaining the potential of an electrostatic latent image which is formed on the surface of a photosensitive member or photoreceptor, constant at all times.
Generally, in a transfer type electrophotographic copying apparatus, for the stabilization of the quality of copied images, it is necessary to maintain the charge potential of the photoreceptor at a predetermined value, i.e.--the surface potential at the image area of an electrostatic latent image and/or the potential at the background thereof must be maintained at a predetermined value, irrespective of the surrounding conditions, operating conditions, etc. of the photoreceptor. It is to be noted that the above surrounding conditions are those affecting the characteristics of the photoreceptor, and will be referred to as such hereinbelow.
In the first place, the present inventors have carried out various experiments with respect to the charge potentials of photoreceptors for maintaining the surface potential at the image area of the electrostatic latent image at a predetermined value, and have ensured that there exists an approximately constant proportional relationship between the surface potential (charge potential) of the photoreceptor and the current flowing through a corona charger, and have thus developed a method therefor as will be explained hereinbelow with reference to FIG. 1. For the experiments referred to above, photoreceptors of Se.Te alloy were employed.
In a graph of FIG. 1, the abscissa represents currents flowing through the corona charger, while the ordinate denotes the surface potential of a photoreceptor drum, with a final target charge potential being represented by (VRE.sub.1). A straight line (A.sub.1) represents the characteristics of the photoreceptor under standard conditions, while another straight line (A.sub.1 ') represents the characteristics thereof during actual use, and the inclination of the lines (A.sub.1) and (A.sub.1 ') may vary in various ways depending on different surrounding conditions.
Accordingly, it is suitable for a general practice to first cause a current with a value (I.sub.0) to flow through the corona charger at an initial stage of charging, while, in correspondence with a detection value (Vm) for the surface potential, a correction current represented by (I.sub.1) [I.sub.1 =I.sub.0 .multidot.VRE.sub.1 /Vm] is caused to flow therethrough, and thereafter, similar corrections are repeated until the detection value of the surface potential reaches the final target charge potential (VRE.sub.1) for subsequently effecting copying process.
However, in the practice as described above, there has been such disadvantages that, since it is required to wastefully move the photoreceptor drum by a distance (l) (FIG. 6) from a charging position to a detecting position, i.e. by a distance equivalent to "the number of corrections x (l)" in total at each time of correction of the current value with respect to the corona charger, without regard to the substantial copying operation, copying speed is undesirably reduced as the number of corrections is increased.
Incidentally, as shown in a graph of FIG. 2, the characteristics of the Se.Te alloy photoreceptor (i.e. the inclination of the line (A.sub.1)) vary in different curves, largely depending on temperature variations of the photoreceptor drum and according to whether the copying apparatus is at an initial stage of its use (solid line curve (B.sub.1)) or it has been used for a long period of time (dotted line curve (B.sub.1 ')).
In connection with the above, in the experiments carried out by the present inventors, it has been ensured that the variations as described above may be approximately represented, at temperatures higher than 25.degree. C., by a quadratic equation EQU K.sub.1 TPC.sup.2 +K.sub.2 TPC+K.sub.3
where TPC is the surface temperature of the photoreceptor, and, at temperatures lower than 25.degree. C., by a simple equation EQU K.sub.4 TPC+K.sub.5
On the other hand, apart from the surface potential at the image area of the electrostatic latent image, the present inventors have also conducted various experiments with respect to potentials of a reference latent image formed on the surface of the photoreceptor and having a potential equal to the background area of the electrostatic latent image, in an attempt to maintain the potential in such background area at a predetermined constant value.
As a result, through utilization of the fact that there is an approximately constant proportional relationship between the potential of the reference latent image and a voltage for an exposure lamp, the present inventors have developed a method therefor as shown in a graph of FIG. 3. In the above case also, the photoreceptors employed in the experiments were of Se.Te alloy.
In the graph of FIG. 3, the abscissa represents exposure lamp voltages (LV), while the ordinate denotes potentials (IV) on the surface of the photoreceptor drum at its portion where the reference latent image is formed, with the final target potential being represented by (VRE.sub.2). A curve (V.sub.2) shows characteristics of the photoreceptor under standard conditions, while another curve (A.sub.2 ') represents characteristics of the photoreceptor during actual use, which may differ depending on various using conditions and surrounding conditions and the like.
The final target potential (VRE.sub.2) referred to above is set at such a potential as will not produce fogging in the copied imges. In the actual copying, since originals in various contrast may be employed, it is necessary to arrange that copied images of various originals are preferably free from formation of fogging. In the experiments as carried out by the present inventors, on the assumption that the reflecting density at the background area of the original image is less than 0.25, a reference latent image forming pattern with the reflection density of 0.25 was employed, while the final target potential (VRE.sub.2) was set at 300 V, with a developing bias fixed at 300 V. It is to be noted that, under the above conditions, the portions of the electrostatic latent image corresponding to the portions of the original image having reflection density less than 0.25, i.e. the background portions of the electrostatic latent image, are not developed.
Accordingly, under certain conditions represented by the curve (A.sub.2 '), a voltage at a value (LV.sub.0) is first applied to the exposure lamp at the initial stage of the reference latent image formation, and in correspondence with the detection value (IVm) for the surface potential, a correction voltage LV.sub.1 represented by ##EQU1## is applied thereto, and thereafter, similar corrections are repeated until the detection value for the surface potential reaches the final target potential (VRE.sub.2) for subsequently effecting the copying process.
It is to be noted that in the above equation, (IVX) is a value set as a constant for convenience, of a surface potential at a point where the ordinate intersects an extension of a tangent line on the final target potential (VRE.sub.2) in the photoreceptor surface potential characteristics under the standard conditions with respect to the exposure lamp voltage (LV) as shown in FIG. 3.
In the above method, however, it is also necessary to wastefully move the photoreceptor drum by the distance (l') (FIG. 13) from an exposing position to a detecting position, i.e. by a distance equivalent to "the number of corrections x (l')" in total at each time of correction of the voltage value with respect to the exposure lamp and irrespective of the substantial copying operation, thus resulting in such disadvantages that the copying speed is undesirably reduced as the number of corrections is increased.
As described earlier, the characteristics of the Se.Te alloy photoreceptor (i.e. the inclination of the curve A.sub.2 and more accurately, the value represented by (IVX-VRE.sub.2)/LV) vary as shown in FIG. 4 in different curves, largely depend on temperature variations of the photoreceptor drum and according to whether the copying apparatus is at an initial stage of its use (curve B.sub.2) or it has been used for a long period of time (curve B.sub.2 ').
In connection with the above, it has also been confirmed in the experiments conducted by the present inventors that the variations as referred to above may be approximately represented, at temperature higher than 25.degree. C., by the quadratic equation EQU K.sub.1 'TPC.sup.2 +K.sub.2 'TPC+K.sub.3 '
where TPC is the surface temperature of the photoreceptor, and, at temperatures lower than 25.degree. C., by the simple equation EQU K.sub.4 'TPC+K.sub.5 '
Meanwhile, it has also been ensured that with respect to the number of repetitions of copying operation within a short period of time also, the characteristics vary in different inclinations between an initial stage of repetition (curve C) and stages after repetition for many times (curve C') as shown in FIG. 5, and that the variations may be approximately represented, at repetitions less than ten times, by a simple equation EQU K.sub.6 '.multidot.log N+K.sub.7 '
and at repetitions more than ten times, by a simple equation EQU K.sub.8 '.multidot.log N+K.sub.9 '